The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It will be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.
The respiratory system of a healthy body facilitates gas exchange. The nose and mouth form the entrance to the airways of a patient, with the trachea and bronchi being the conducting airways to take the air to the alveolated region of the lungs where the gas exchange takes place. See for example “Respiratory Physiology”, by John B. West, Lippincott Williams & Wilkins, 9th edition published 2011.
For example, a respiratory pressure therapy (RPT) device delivers pressurized air to a patient's airway, acting as a splint to keep the airways open, allowing the patient to breathe normally when asleep. The pressurized air is delivered in quantities beyond that required for respiration, with the excess flow allowed to leak out a mask vent so that therapeutic pressure is maintained. RPT is used to treat sleep apnoea and other respiratory disorders. If this flow of air is not humidified, the patient's airways can dry out, causing discomfort to the patient.
The use of a humidifier is intended to produce humidified gas that minimizes drying of the nasal mucosa and increases patient airway comfort. Respiratory humidifiers are available in many forms, and for example may be a standalone device that is coupled to an RPT device via an air conduit; integrated within an RPT device; or be configured so that it is operatively directly coupled to an RPT device.
Humidifiers typically comprise a water tub having a capacity of several hundred milliliters (ml), a heating element for heating the water in the tub, a control to enable the level of humidification to be varied, a gas inlet to receive gas from the RPT device, and a gas outlet adapted to be connected to an air circuit that delivers the humidified gas to the patient interface. In an RPT application, the water tub should contain more than enough water to last for the sleep duration of the patient.
Heated pass-over humidification is one common form used with RPT devices. In such humidifiers, the heating element is typically incorporated in a heater plate which sits under, and is in thermal contact with, the water tub. Heat is transferred from the heater plate to the water tub primarily by conduction. The air flow from the RPT device passes over the heated water in the water tub, resulting in water vapour being taken up by the air flow.
The tub contains the entire volume of water to be used to humidify the flow of air, and receives the flow of air which passes over the water and thereby delivers the humidified flow of air. Such a humidifier configuration presents a number of drawbacks. There is a risk of spillage of the volume of water into the RPT device or to the patient. The entire volume of water must be heated, with a consequent long warm up time and cool down time, of the order of 20 minutes at best. Also, as the tub becomes empty, any contaminants in the water will be adhered as residue in the tub, which will then require cleaning.
Other humidifiers use wick type arrangements, whereby water is drawn through the wick, and the air flows across the surface of the wick, thereby transferring water to the air flow. Water is generally pumped from the reservoir to the wick. A drawback with such wick systems is that the water reservoir is generally at ambient pressure and the water is fed by pump to the wick which is enclosing the pressurized air flow. Thus the pump or water delivery mechanism must act as a valve to prevent loss of air flow and/or pressure, and the pump must deliver a regulated quantity of water to the wick so that flooding does not occur, but still maintain sufficient water in the wick for adequate humidification.
US2009/0000620 (granted as U.S. Pat. No. 8,550,075) discloses a humidifier which includes a water reservoir and a semipermeable membrane on top, to allow diffusion of water vapour. This humidifier requires a complex, specific structure. In particular, it requires a water distribution member comprising an envelope formed by a first compartment wall and a second compartment wall joined together, the water distribution member being supported by a base plate, a heater apparatus supported on the base plate, and (in effect) underneath the water distribution member. Thus, the entire volume of water in the reservoir is heated.
WO2015/135040 discloses a humidifier comprising a reservoir and a pump to provide a flow of water to a humidifier wick, the humidifier wick including a heating element and having a profiled shape so as to enclose at least part of the air flow path through the humidifier.
It is an object of the present invention to provide a more cost effective humidifier and method of humidification.